Process for pyrolyzing tetrafluoroethylene to hexafluoropropylene

ABSTRACT

A feed mixture of tetrafluoroethylene and a minor proportion of carbon dioxide is fed to a pyrolysis furnace operating at about atmospheric pressure and a temperature of 700* to 900*C., and a conversion of 20 to 80 percent by weight, to a yield of at least 80 percent by weight of hexafluoropropylene and a mixture of carbon dioxide and unreacted tetrafluoroethylene which is distilled from the hexafluoropropylene. The distillate mixture can be recycled to the pyrolysis furnace.

United States Patent [ll] 3,873,630 West Mar. 25, 1975 [54] PROCESS FORPYROLYZING 2,759,983 8/1956 Waddell 260/6533 TETR FLU R ETH LENE T2,970,176 2/1961 Ten Eyck et al. 260/6533 rs g g z 0 3,009,966 11/1961Glenside et al. 260/6533 3,519,384 7/1970 Engel et a1 423/432 [75]Inventor: Norman Eugene West, Vienna, W, 3,578,721 5/1971 Couture260/6533 Va. 73 Assignee: E. 1. DuPont de Nemours and j 'r g r" 33 rCompany, Wilmington DeL SSIS an .iammer am s me [22] Filed: June 12,1972 [57 ABSTRACT [21] Appl. No.: 261,629 A feed mixture oftetrafluoroethylene and a minor proportion of carbon dioxide is fed to apyrolysis furnace operating at about atmospheric pressure and a [52] US.Cl. 260/6533, 260/6536 temperature of to 900C and a Conversion of 20[51] Int. Cl. C07c 21/20 [58] Fi Id of 5 Nb 260/653 3 to 80 percent byweight, to a yeild of at least 80 pere ea cent by weight ofhexafluoropropylene and a mixture [56] Refere ces Cited of carbondioxide and unreacted tetrafluoroethylene n which is distilled from thehexafluoropropylene. The UNITED STATES PATENTS distillate mixture can berecycled to the pyrolysis fur- 2,394,58l 2/1946 Benning ct 211 260/6533nace, 2,551,573 5/1951 Downing et al 260/6533 2,758,138 8/1956 Nelson260/6533 6 Chums N0 Drawmgs PROCESS FOR PYROLYZING TETRAFLUOROETHYLENETO HEXAFLUOROPROPYLENE This invention relates to pyrolysis of tetrafluoroethylene to hexafluoropropylene.

U.S. Pat. No. 2,551,573 to Downing et al. discloses the pyrolysis ofCHCIF to tetrafluoroethylene and higher boiling fluorocarbon products.U.S. Pat. No. 2,758,138 to Nelson discloses the pyrolysis oftetrafluoroethylene to hexafluoropropylene at various specificconditions involving low partial pressures, i.e., 25 to 200 mm. of Hg,of the tetrafluoroethylene feed. A process for pyrolyzingtetrafluoroethylene to hexafluoropropylene at higher pressures, viz.,0.2 to 65 psia. is disclosed in U.S. Pat. No. 2,970,176 to Ten Eyck etal., wherein the tetrafluoroethylene feed contains at least weightpercent of higher boiling fluorocarbon compounds which contribute to theformation of hexafluoropropylene. A more economical process wasdeveloped later in which CHClF is pyrolyzed to both tetrafluoroethyleneand hexafluoropropylene by conducting the pyrolysis within a specificconversion range of 86 to 94 percent as described in U.S. Pat. No.3,306,940 to Halliwell.

The present invention provides a pyrolysis process for makinghexafluoropropylene which is economically advantageous over all theforegoing described processes. The process involves pyrolyzing a feed oftetra tluoroethylenc (TFE) and a minor proportion of car bon dioxide ina reaction zone at a temperature of 700 to 900C, and a pressure of 0.75to 2.0 atmospheres absolute, and a minimum partial pressure of TFE of360 mm. ofHg absolute, to a conversion ofTFE at least of percent byweight, separating the unpyrolyzed TFE and the carbon dioxide from thehexafluoropropylene (HFP) and other pyrolysis products produced, and,ifdesired, recycling the separated TFE and carbon dioxide to thereaction zone while adding make-up TFE to maintain a substantiallyconstant proportion of TFE in the feed to the reaction zone. The processhas been operated economically at conversions of TFE up to about 80percent. The major pyrolysis product, i.e., greater than 50 percent byweight of the pyrolysis products, is HFP. Yields of greater than 80percent by weight and as high as 90 percent by weight of TFE to HFP, thedesired product, can be obtained by this pro cess. When the unpyrolyzedTFE and carbon dioxide are recycled to the reaction zone, then steps toisolate the carbon dioxide from the TFE are unnecessary.

A major problem involved in pyrolyzing TFE is its tendency todisproportionate to carbon tetrafluoricle and carbon according to thereaction which represents both a yield loss of expensive chemical andcauses plugging of the reaction apparatus. This disproportionation is anexothermic reaction so once it begins, it tends to be a runawayreaction. The invention of the aforementioned Nelson patent attempted toavoid this problem by operating at very low pressures, i.e., less than200 mm. of Hg absolute. This low pressure of operation was obtained bycontinuously evacuating the pyrolysis furnace and feeding the TFE to thefurnace, which, by itself, determined the pressure in the furnace. Onedisadvantage of this vacuum operation was the danger of leakage of airinto the pyrolysis systern from the surrounding atmosphere which wouldresult in oxidation of TFE and could give an explosive mixture of air inTFE. Another disadvantage was the need for using larger equipment forthe same throughput achieved at higher pressure operations, such asdisclosed in the Downing et al. patent.

The invention of the Ten Eyck et al. patent found that TFE could bepyrolyzed to HFP at higher, more conveniently obtainable pressures suchas atmospheric pressure, but this required the presence of at least 5percent of pyrolyzable fluorocarbon materials having a boiling pointhigher than HFP which were normally provided by recycling the higherboiling pyrolyzate, which had the disadvantage of repeated handling ofhighly toxic materials rather than leading to quick waste disposalthereof.

The present invention also operates at the more favored pressurecondition of around atmospheric pressure but without requiring thepresence of the highboiling fluorocarbon compounds and the attendanttoxicity problem and yet avoiding the disproportionate problem.

The process of the present invention is most conveniently conductedpreferably at atmospheric pressure by passing a mixture of the TFE andcarbon dioxide through a heated tube, called a pyrolysis furnace, withthe heated portion of the tube being the reaction or pyrolysis zone. Thepresence of the carbon dioxide in the pyrolysis furnace during thepyrolysis of TFE in the present invention prevents the undesirabledisproportionation ofthe TFE despite the partial pressure ofTFE beingsubstantially higher than that disclosed by Nelson.

With respect to the proportions of TFE and carbon dioxide in the feed tothe reaction Zone (pyrolysis furnace), there is always a greater weightof TFE present than carbon dioxide, and thus the latter is present as aminor proportion (weight basis) of the feed. Typically the proportionofTFE to carbon dioxide in the feed will be from 1:1 to 25:1 on a weightbasis. At proportions of TFE/CO of greater than 25:1, it is feared thatlongterm repression of the disproportionation reaction will not occur.The feed mixture to the reaction zone advantageously contains at least40 percent by weight (total feed basis) of carbon dioxide to render themixture nonflammable in case of leakage to the atmosphere. Preferably,the partial pressure of TFE in the feed mixture is at least 450 mm. ofHg absolute.

lmmediately succeeding the pyrolysis zone of the furnace, gaseoushydrogen chloride or hydrogen fluoride is preferably injected into thepyrolysis mixture as described in U.S. Pat. No. 3,578,721 to Coutureeta1., in order to prevent polymerization of unreacted TFE which wouldeventually cause plugging of the recovery system.

The next step in the process is to recover the unreacted TFE and carbondioxide as a mixture from the rest of the pyrolysis mixture bytechniques well known in the art, such as distillation. The boilingpoint of the TFE is 76.8C. and the sublimation point of carbon dioxideis -78C. which enables these compounds to be separated from the HFP(b.p. 29C) and higher boiling fluorocarbon pyrolysis products in asingle distillation operation and recovered as a mixture. Thedistillation is conducted under pressure, such as 17.5 kg/cm (gauge), toraise the boiling point of the distillate to a convenient temperature ofoperation, which temperature is above the temperature at which solid COcould exist in the distillation system. If HCl is added to thepyrolyzate, as described above, it will distill with the TFE as anazeotrope therewith. 1f the additive is HF. it will a mixture to thelnconel tube via calibrated rotameters. Hydrogen chloride was injectedinto the exit zone of the pyrolysis tube where the temperature was about500C. to inhibit polymerization of unreacted TFE in not dlstlll with theTFE but will remain with the HFP 5 the collection system. The lnconeltube was heated to and other hlgh'bolllhg Py y productswhen the thedesired reaction temperature with a carbon dioxide HCl is distilled withthe TFE and carbon dioxide, the purge d h TFE was added to the feedstream to resultant gaseous mlxlhle can be Passed through a the tube toobtain a mixture having the desired TFE/- ventional water-scrubber toremove the HCl to prevent (IQ ti Th y ol sis was conducted at aboutatmolhe l'lcl from behlg recycled to the py y furnace l sphericpressure. Samples of the exit gas from the tube with the recycled Carbodi ide d were scrubbed in water to remove HCl and analyzed by ll the Hclwere to be recycled to the Py y gas chromatography to establish yieldsand converhaee lhefe would be a Yield loss because llCl would be sions.Further details of these experiments and the rereaCtive ith TFE and i hth ifluqroqa rhene 5 1 1: sults are shown in the following Table:

HCl feed TFE Yield to Exam- Temp. Contact rate TFE/CO Conversion HFP pleC. time-sec. ml/min. wt.ratio 7r cals formed in the pyrolysis reaction.1f the polymeriza- These results show high yields to HFP at various tioninhibitor is HF, it will have already been removed TFE/carbon dioxidefeed ratios and other variations in from the TFE/CO mixture in the HFPdistillation step. process conditions.

If desired, the carbon dioxide can be separated from the TFE by passingthis mixture through a conventional EXAMPLES 7 to 10 hqhews when:scrubbef- Bmh f c and carbon w These experiments were conducted in alarger pyrol. Oxldg can be separated m h 1 FE m h h l ysis furnace inwhich the lnconel tube was 1.78 cm. in by aqueous caustic scrubbing, ifthe TFE is desired for diameter and 487 Cm long, with the thermocouplcsome purpose otheruthim recycle monomer for being installed inside thetube and near its exit end. subsequent polymerization to valuable polymand with the procedure including pressure being essentially Copolymers'I the same as in Examples 1 to 6. The following pyrolysis The HFP can beseparated by dlstlllatlon from the conditions were used and results wereobtained. higher-boiling pyrolysis products. The HFP is useful in thesame manner as HFP synthesized by other techniques heretofore to makeknown copolymers, e.g., with TFE, or chemicals such as HFP epoxide. TFEYield to The process is normally conducted on a continuous 40 ConversionH ZP basis, i.e., continuously passing the feed mixture p through thereaction zone, followed by continuous sep- 7 319 907 aration, scrubbingand recycling. 3 $33 38 gig $313 Specific embodiments of the process ofthe present 0 7 6 1.0 1.54 28.8 89.6 invention are as follows: theseembodiments are intended as examples of the present invention and not asHT -AT i MT an M a llmltatlon on the Scope h In these e P i Theseexperiments were operated for 22 hours with 2? ig gg bylwellghtdunlesz li i no problem in temperature control being encountered, cate. ie toiscacuate onte asistate cyclic dimer can be recovered as a valuablefluorocar bon chemical. or it can be pyrolyzed to HFP as is dis-EXAMPLES 11 to 12 closed in U.S. Pat. No. 3,306,940 to Halliwell. Theco-distillation of TFE, CO and HCl and separation from HFP and thehigher boiling fluorocarbon py- EXAMPLES l to 6 55 rolysis products canbe done as follows: The pyrolyzate, A series of pyrolysis experimentswas conducted for example that obtained from the run illustrated byusing the following equipment and procedure: The py- Example 8, iscompressed and fractionally distilled to rolysis furnace consisted of atube of lnconel metal recover an overhead mixture consisting ofunreacted (nickel-chromium-iron alloy with small amounts of sili- TFE,CO and HCl boiling at -22C. at a pressure of con); the tube had adiameter of 0.635 cm. and heating 14.7 kg/cm The desired product HFP isrecovered length of about 30.5 cm. to give a volume ofthe heated fromthe tails stream of the column. zone of about 5 cc. The tube was heatedby a 2.54 cm. The recycle of carbon dioxide and unreacted TFE todiameter tubular furnace positioned about the heated the pyrolysisfurnace can be done as follows: The preslength ofthe lnconel tube. Thetemperature of the tube sure of the TFE, CO and HCl distillate of thepreced was measured using a chromel-alumel thermocouple attached to theoutside of the tube near the center of the heated zone. TFE and carbondioxide were fed as ing paragraph is reduced to atmospheric pressure,and the distillate is scrubbed in water to remove HCl and is dried overCaSO The composition of the resultant TFE and CO mixture is 39 percentTFE by weight and 61 percent CO by weight. To 1 part of the product isadded 0.41 parts of makeup TFE to give a mixture of TFE and CO having aTFE/CO weight ratio of 1.31. This mixture is fed to a heated lnconeltube at a rate to give the contact times and results shown in Examples 1l and 12; the contact time being defined as the volume of the reactordivided by the volumetric flow of the ambient temperature feed persecond.

TFE Yield to Exam- Temp. Contact TFE/CO Conversion HFP ple C. time-sec.wt.ratio A Wt.7r

percent of the TFE to hexafluoropropylene (HFP), with injection of amaterial selected from the class of HCl or HF into the pyrolysis mixtureto prevent polymerization of unconverted TFE, followed by cooling andseparation of HFP, and recycling of unconverted TFE with makeup TFE; theimprovement which comprises diluting the feed to the pyrolysis with COat a TFEzCO weight ratio of 1:1 to 25:1 so the TFE partial pressure isat least 360 mm Hg absolute and the total pressure is 0.75 to 2.0atmospheres absolute.

2. The process of claim 1 wherein HCl or HF is separated from the cooledproduct mixture by water scrubbing, the HFP is separated from the TFE-COmixture by distillation, and the TFE-CO mixture remaining is recycled.

3. The process of claim 1 wherein HCl or HF and CO is separated from thecooled product mixture by aqueous caustic scrubbing, the HFP isseparated from the TFE by distillation, and the TFE remaining is recycled.

4. The process ofclaim 1 wherein at least 40 percent by weight of saidfeed mixture is carbon dioxide.

5. The process of claim 1 wherein the pyrolysis is conducted to aconversion of tetrafluoroethylene of 20 to percent by weight.

6. The process of claim 5 wherein the pyrolysis is conducted to a yieldof hexafluoropropylene of at least 80 percent by weight.

1. IN A PROCESS FOR PYROLYZING IN A REACTION ZONE A FEED MIXTURE OF TETRAFLUOROTHYLENE (TFE) AT A TEMPERATURE BETWEEN 700* AND 900*C. TO CONVERT AT LEAST 20 PERCENT OF THE TFE TO HEXAFLUOROPROPYLENNE (HFP), WITH INNJECTION OF A MATERIAL SELECTED FROM THE CLASS OF HC1 OR HF INTO THE PYROLYSIS MIXTURE TO PREVENT POLYMERIZATION OF UNCONVERTED TFE, FOLLOWED BY COOLING AND SEPARATION OF HFP, AND RECYCLING OF UNCONVERTED TFE WITH MAKEUP TFE, THE IMPROVEMENT WHICH COMPRISES DILUTING THE FEED TO THE PYROLYSIS WITH CO2 AT A TFE:CO2 WEIGHT RATIO OF 1:1 TO 25:1 SO THE TFE PARTIAL PRES-S SURE IS AT LEAST 360 MM HG ABSOLUTE AND THE TOTAL PRESSURE IS 0.75 TO 2.0 ATMOSPHERES ABSOLUTE.
 2. The process of claim 1 wherein HCl or HF is separated from the cooled product mixture by water scrubbing, the HFP is separated from the TFE-CO2 mixture by distillation, and the TFE-CO2 mixture remaining is recycled.
 3. The process of claim 1 wherein HCl or HF and CO2 is separated from the cooled product mixture by aqueous caustic scrubbing, the HFP is separated from the TFE by distillation, and the TFE remaining is recycled.
 4. The process of claim 1 wherein at least 40 percent by weight of said feed mixture is carbon dioxide.
 5. The process of claim 1 wherein the pyrolysis is conducted to a conversion of tetrafluoroethylene of 20 to 80 percent by weight.
 6. The process of claim 5 wherein the pyrolysis is conducted to a yield of hexafluoropropylene of at least 80 percent by weight. 